Most commercial lower-extremity prostheses and orthoses are passive and cannot provide positive mechanical power to replicate joint biomechanics during the gait cycle. Existing approaches to the design of powered knee systems have focused mainly on the use of single motor-transmission systems directly coupled to the joint. Such direct-drive designs, however, require high electrical power consumption in order to fully emulate the mechanical behavior of the biological knee joint even during level-ground ambulation. One reason for this lack of energetic economy is inadequate use of the passive dynamics of the leg, and elastic energy storage and return of tendon-like structures.
Knee prostheses for above-knee amputees can be classified into three major groups: passive, variable-damping, and powered. Passive prosthetic knees do not require a power supply for their operation, and are generally less adaptive to environmental disturbances than variable-damping prostheses. Variable-damping knees do require a power source, but only to modulate damping levels, whereas powered prosthetic knees are capable of performing non-conservative positive knee work.
Variable-damping knees offer several advantages over mechanically passive designs, including enhanced knee stability and adaptation to different ambulatory speeds. Although variable-damping knees offer some advantages over purely passive knee mechanisms, they are nonetheless incapable of producing positive mechanical power and therefore cannot replicate the positive work phases of the human knee joint for such activities as sit-to-stand maneuvers, level-ground walking, and stair/slope ascent ambulation. Not surprisingly, transfemoral amputees experience clinical problems when using variable-damping knee technology, such as, for example, asymmetric gait patterns, slower gait speeds, and elevated metabolic energy requirements compared to non-amputees.